With the development of orthogonal frequency division multiplexing (OFDM) systems and the deployment of optical fiber transmission lines, base stations of Worldwide Interoperability for Microwave Access (WiMax) communication systems, 3GPP Long Term Evolution (LTE) communication systems and other OFDM mobile communication systems have gradually evolved from co-located antenna systems (CASs) into distributed antenna system (DASs). As compared to co-located antenna systems, distributed antenna systems can extend the signal coverage, lower the deployment cost of base stations and reduce the overall signal transmission power. Therefore, distributed antenna systems have become a development tendency for the deployment of base stations.
A conventional TDD-OFDM distributed antenna system 1 is as shown in FIG. 1. The distributed antenna system 1 comprises a base station 11 (as known as a centralized plant or a headend) and a remote access unit (RAU) 13. The base station 11 connects to the RAU 13 via a fiber transmission line 15. The base station 11 comprises a signal processing circuit 111, an FDM BP filter bank 113, a control circuit 115 and a fiber transceiver 117. The signal processing circuit 111 receives a user downlink signal 102 from a backhaul network and performs an inverse fast Fourier transform on the user downlink signal 102 to generate a first OFDM signal 104. The FDM BP filter bank 113 receives a clock synchronization signal 106 from the control circuit 115 and an input control signal 108 carried on a specific band.
Then, the fiber transceiver 117 receives the first OFDM signal 104, the clock synchronization signal 106 and the input control signal 108, combines them with each other, and transmits them to the RAU 13. The RAU 13 comprises a radio frequency (RF) transceiver 131, an FDM BP filter bank 133, a fiber transceiver 135, a synchronization circuit 137 and a control circuit 139. The fiber transceiver 135 receives and transmits the first OFDM signal 104, the clock synchronization signal 106 and the input control signal 108 to the RF transceiver 131 and the FDM BP filter bank 133. It shall be noted that in practical operations, the fiber transceiver 117 and the fiber transceiver 135 convert electric signals to be transmitted (e.g., the first OFDM signal 104 and the input control signal 108) into optical signals for transmission through fiber transmission lines, and convert optical signals that are received into electric signals. However, for the purpose of simplicity, the method in which the fiber transceivers transmit and receive signals and the conversions between the electric signals and optical signals will not be described herein.
After receiving the first OFDM signal 104, the clock synchronization signal 106 and the input control signal 108, the RF transceiver 131 detects the energy of the received signals (i.e., the total energy of the first OFDM signal 104, the clock synchronization signal 106 and the input control signal 108). Based on the time in which the energy of the received signals is greater than the preset value, the RF transceiver 131 switches between a transmitting mode and a receiving mode periodically. In the transmitting mode, the RF transceiver 131 transmits the first OFDM 104 via an antenna (not shown) but filters out other signals than the first OFDM signal 104.
On the other hand, after receiving the first OFDM signal 104, the clock synchronization signal 106 and the input control signal 108, the FDM BP filter bank 133 retrieves the clock synchronization signal 106 from the specific band and transmits the clock synchronization signal 106 to the synchronization circuit 137, and retrieves the input control circuit 108 from the specific band and transmits the input control circuit 108 to the control circuit 139. Then, the synchronization circuit 137 performs a clock synchronization with the base station 11 according to the clock synchronization signal 106, and the control circuit 139 performs a system configuration according to the input control signal 108. For example, the input control signal 108 may comprise gain control parameters for a power amplifier, gain setting parameters for a low-noise amplifier, receiving filter band options, phase lock loop (PLL) frequency options or other system setting parameters.
In the receiving mode, the RF transceiver 131 receives a user uplink signal 110 via an antenna (not shown) and transmits the user uplink signal 110 to the fiber transceiver 135. At this point, an output state signal 112 carried on the specific band is generated by the control circuit 139 as a state reported for network management. The output state signal 112 comprises a PLL lock detection state, a temperature sensor output state and other system states. The output state signal 112 is transmitted to the fiber transceiver 135 via the FDM BP filter bank 133. The fiber transceiver 135 receives the user uplink signal 110 and the output state signal 112, combines the user uplink signal 110 and the output state signal 112 together, and transmits them to the base station 11.
The fiber transceiver 117 of the base station 11 receives and transmits the user uplink signal 110 and the output state signal 112 to the signal processing circuit 111 and an FDM BP filter bank 113. The signal processing circuit 111 filters out the signals other than the user uplink signal 110, performs a fast Fourier transform on the user uplink signal 110 and transmits the transformed user uplink signal 110 to the backhaul network. The FDM BP filter bank 113 retrieves the output state signal 112 from the specific band and transmits the output state signal 112 to the control circuit 115 and the backhaul network.
As can be known from the above description, because the received signals in the conventional TDD-OFDM distributed antenna system belong to analog signals, they are liable to attenuation and interferences during transmission; consequently, the reliability of the preset values can only be ensured through an adaptive setting. Furthermore, because filtering must be performed by the FDM BP filter bank 113 on the clock synchronization signal 106 and the input control signal 108 before they are transmitted and on the output control signal 112 after it is received, the hardware cost of the FDM BP filter bank 113 is inevitable for the base station 111.
Accordingly, an urgent need exists in the art to improve the mechanism of switching the RAU between the transmitting mode and the receiving mode and to lower the hardware cost of the base station.